Wood pulp forming system and method of recovering spent chemicals

ABSTRACT

Sorbed sodium ions in the wood pulp are recovered in the washing operation following cooking with the minimum use of pH control acid, thereby reducing cost and any possible undesirable side effects. A washing press is used in the pulp process. Means are provided for adding an acidifying chemical to the washing press for controlling pH of the pulp suspension. The filtrate from the washing press is divided, with the larger portion of the filtrate being fed to the pulp suspension line to the washing press for pulp suspension pH control. A smaller portion of the filtrate is fed countercurrently through the brown stock washing system. The chemicals are then recovered from the brown stock washing system filtrate.

This invention relates to wood pulp processes and equipment. Moreparticularly, this invention is a new system and method for recoveringthe spent chemicals resulting from cooking wood in a digester of a pulpprocessing system.

It is well known that cellulose has an affinity for metallic ions whichis a function of the pH of the surrounding and internal water. The moreacetic, or the lower the pH, the water is surrounding and in theinterstices of the fiber, the lower the affinity; and the more alkalineor the higher the pH, the higher the affinity. Thus, more metallic ions,mostly sodium, contained in the chemicals used in a digester are sorbedon the fibers of the wood when the wash water has a higher pH than whenthe wash water has a lower pH. Thus, if all other factors are the same,less chemical may be recovered for re-use at a higher pH than at a lowerpH. Of course, certain pH ranges must be kept at different areas in apulp plant but wherever reasonably possible, the lowering of the pH ofthe pulp will increase the amount of chemical which can be recovered.These chemicals are expensive.

Briefly described, my new method for forming pulp from wood comprises akraft batch or continuous digester, a brown stock washing system and awashing press. Means are provided for feeding cooked pulp to the brownstock washing system, and means are provided for feeding the pulpsuspension from the brown stock washing system to the washing press.Means are provided for feeding a washing liquid to the washing presswhich washing liquid has had an acidifying chemical added to it. Meansare provided for countercurrently flowing the filtrate from the washingpress through the brown stock washing system and also to the pulpsuspension line from the brown stock washing system to the washingpress. The chemicals are recovered from the brown stock washing systemfiltrate.

Briefly described, the new method of recovering spent chemicals used incooking wood in a digester comprises the steps of feeding the pulpsuspension through a brown stock washing system and then to a washingpress. The wash liquid is acidified before it is fed to the washingpress to provide a pulp suspension pH at which the optimum amount ofmetallic, most importantly sodium ions are desorbed, solubilized, andmade recoverable. A portion of the acid containing filtrate from thewashing press is recycled back to the pulp suspension being fed to thewashing press and the remaining or excess portion is recycledcountercurrently through the brown stock washing system. The chemicalscontained in the brown stock washing system filtrate are recovered.

The invention, as well as its many advantages, may be further understoodby reference to the following detailed description and drawing which isa single FIGURE showing a schematic flow diagram of one preferred systemfor forming pulp from wood and method of recovering the spent chemicals.

Referring to the schematic flow diagram, wood, usually in the form ofwood chips, contained in the digester 10 is cooked in the presence ofvarious chemicals, the primary metallic ion in the chemicals beingsodium. The pulp is formed by dissolving the lignin and other materialof the wood with the chemicals thus freeing the cellulose fibers so thatthey can be reformed into paper.

A mixture of the wood and a liquor is steam heated in the digester 10under pressure. Cellulose fibers (pulp) and spent cooking solution isthen fed by means of line 12 to a brown stock washing system 14. In thebrown stock washing system 14, the majority of the spent cookingsolution or black liquor is washed out of the pulp. The filtrate fromthe brown stock washing system contains most of the cooking liquorchemicals and the dissolved portion of the wood.

From the brown stock washing system 14, the pulp is fed by means of line16 to a press feed tank 18, and then to a washing press 20 by means of apulp line 22.

The wash liquid for the washing press 20 is fed to the washing press bymeans of the line 24. Acidifying chemicals are added to the wash liquidin line 24 by means of line 26 controlled by valve 28.

The amount of acidified chemicals added to line 24 is controlled toprovide a pulp suspension pH from the washing press 20 and in the pulpline 30 which is compatible with the pH required for optimum removal ofsorbed sodium on the pulp.

The acid containing filtrate from the wash press 20 is fed by means oflines 32, 34 and 36 to a filtrate surge tank 38. A portion of the acidcontaining filtrate is recycled back to the pulp suspension being fed tothe washing press 20. This may be done by means of line 40 leading fromthe filtrate surge tank 38 to the press feed tank 18. The remainder orexcess portion of said filtrate is fed counter-currently through thebrown stock washing system 14 by means of line 42 and also contributesto pH reduction of the pulp suspension. The filtrate from the brownstock washing system 14 is fed through line 44 to a chemicals recoveryand cooking liquor regeneration system 46. Should a continuous digesterbe used for cooking and it contains a diffusion washing zone, filtratefrom the washing system line 44 is diverted by line 48 to the digesterand continues on to the recovery system via line 50.

Note particularly that a pulp press 20 is used rather than a vacuumfilter or a diffusion washer. By using a pulp press, rather than avacuum filter or a diffusion washer the sodium recovery process is moreefficiently performed. More specifically, the lowering of the pulp pH isaccomplished at a considerable savings in acids used for pH adjustmentof the alkaline wash solution. For the same amount of liquid from thedigester 10 per unit weight of oven dry pulp produced, almost threetimes the weight of water per unit weight of pulp must be added to adiffusion washing system and more than two times the weight of water perunit weight of pulp must be added to a vacuum filter system than when awashing press is used as the final step of washing.

This system and method has several advantages over any method and systemknown to date by those skilled in the art and science of pulp washingand removal of normally unwashable absorbed chemicals. The amount ofacidifying chemical is less when treating fresh water or evaporatorcondensate to a constant or controlled pH. By virtue of the low watercontent of the discharged pulp from the washing press 20, a lower pHpulp can be achieved per unit of acidifying chemical than when the feedin line 22 is treated as described by the prior art.

Reducing the amount of acidifying chemicals recycled countercurrently isdesirable. Reduced acid usage provides for a more gradual decrease in pHas the acid effect moves countercurrently through the system, therebyreducing to a minimum any undesirable side effects such as ligninprecipitation or acid anion buildup in the closed chemical recoverysystem.

I claim:
 1. A method of recovering spent chemicals used in cooking woodin a digester comprising the steps of: feeding the pulp suspensionthrough a brown stock washing system, and then to a washing press;acidifying the wash liquid before the wash liquid enters the washingpress to provide a pulp suspension pH from the washing press compatiblewith desorbtion of metallic ions; recycling one portion of the acidcontaining filtrate back to the pulp suspension being fed to the washingpress and recycling the remaining portion of said acid containingfiltrate countercurrently through the brown stock washing system; andrecovering the chemicals contained in the brown stock washing systemfiltrate.
 2. A system for forming pulp from wood comprising: a kraftcontinuous digester; a brown stock washing system; a washing press;means for feeding cooked pulp from the digester to the brown stockwashing system; means for feeding a pulp suspension from the brown stockwashing system to the washing press; means for feeding washing liquid tothe washing press; means for adding an acidifying chemical to the meansfor feeding the washing liquid to the washing press; means forcountercurrently flowing the filtrate from the washing press through thebrown stock washing system; and means for recovery of chemicals from thebrown stock washing system filtrate.